1. Technical Field
The present invention relates to an image forming apparatus such as a copier, a printer, a facsimile machine, a plotter, and a multifunctional apparatus having one or more capabilities of the above devices, and more specifically, relates to an image forming apparatus including a developer container to stir and mix a developer and circulate the developer therein, separately from a developing device to render an electrostatic latent image formed on an image carrier visible.
2. Related Art
In an image forming apparatus employing an electrophotographic method, provision of the developer container to circulate the developer separately from a developing device to render an electrostatic latent image carried on an image carrier visible allows the developing device to be made compact. Moreover, the provision of a developer stirring member with excellent stirring capability may improve mixture and dispersion of toner to be replenished to the developer contained in the developer container, thereby providing stable image development with the toner having a constant density and charge at a high speed and obtaining a high quality image. Such a technique is disclosed in JP-2009-116198-A, which enables formation of a compact developing device and is suitable for multicolor formation with multiple imaging stations. However, the technique also has the disadvantage of reduced heat dissipation because the developing device has a smaller surface area.
In an image forming apparatus employing an electrophotographic method, an image formed of toner is created on a recording sheet such as a sheet of paper through a series of various processes, from charging, exposure, development, and transfer to fixing. In these processes, heat is generated from various parts and components such as motors, light sources, fixing devices, and the like.
In the developing device itself, heat is generated by a difference in linear speed between a photoreceptor and a developing roller, by an eddy current produced by high-speed rotation of a developing sleeve around a magnet, at a shaft bearing, from friction with a doctor blade when spreading the developer on the developer carrier, and the like. Thus, the developing device itself is a source of heat.
For these reasons, the temperature inside the image forming apparatus increases with continuous printing, changing the properties of the toner inside the developing device, thereby creating various problems, such as a decrease in the fluidity of the developer and agglomeration of the toner. Such problems may prevent proper image formation.
To cope with these problems, JP-2009-116198-A discloses a technique in which air is taken in from an exterior of the image forming apparatus via a fan in order to reduce an increase in the temperature of the developer, the developer discharged from the developer container is conveyed by air pressure to the developing device using the external air having a lower temperature, and the developer is cooled while being conveyed. However, because sufficient cooling time cannot be obtained through such a method, a longer conveyance path of the developer becomes necessary, leading to a reduction in path conveyance efficiency.
JP-2011-150243-A discloses a technique to cool the developer container by disposing the developer container separately from the developing device. When the developer container is cooled, the developer inside can be effectively cooled without the conveyance path to convey the developer being lengthened.
FIG. 5 of the same publication discloses a system to cool the developer container by providing a coolant conveyance path around the exterior of the developer container and by passing the external air through the coolant conveyance path. The developer container is formed of a material with a higher thermal conductivity to move the heat of the developer inside the container swiftly to the outside, where the heat is dissipated by air contacting the exterior of the developer container.
However, because the air as a coolant is itself heated by heat generated at portions of the image forming apparatus other than the developing device, the effectiveness of this system decreases over time.